The invention relates to improvements in gas flow measurement, and more specifically to use of critical flow nozzles for improved accuracy and/or for gas flow measurement at high mass flow rates.
By way of background, the measurement and control of the mass flow of gases is important in a wide variety of industrial processes. This includes the control of reactant gases used in semiconductor processes and pharmaceutical applications as well as the precise measurement of gases to create known gas blends. A variety of devices exist to measure and control mass flow and these devices need to be regularly calibrated with reference standards to assure their accuracy. The potential accuracy of the devices is continually improved in response to process needs and therefore the accuracy of the standards available to calibrate them must continually improve.
Perhaps the closest prior art includes the American National Standard entitled xe2x80x9cMeasurement of Gas Flow by Means of Critical Flow Venturi Nozzlesxe2x80x9d, ASME/ANSI MFC-7M-1987 et ISO 9300:1990 sponsored and published by The American Society of Mechanical Engineers, incorporated herein by reference, and commonly assigned patent U.S. Pat. No. 5,445,035 entitled xe2x80x9cPRECISION GAS MASS FLOW MEASUREMENT APPARATUS AND METHOD MAINTAINING CONSTANT FLUID TEMPERATURE IN THIN ELONGATED FLOW PATHxe2x80x9d issued Aug. 29, 1995 to Pierre R .Delajoud, also incorporated herein by reference.
The above mentioned U.S. standard, hereinafter referred to as xe2x80x9cthe U.S. standardxe2x80x9d, discloses critical flow nozzles, in which the gas pressure upstream of the nozzle""s throat is great enough relative to the downstream pressure to ensure that the gas flow velocity at the throat reaches xe2x80x9ccritical flowxe2x80x9d or xe2x80x9csonic flowxe2x80x9d, i.e., reaches the local value of the speed of sound (acoustic velocity). The speed of sound in a critical flow nozzle is a limiting speed that the gas flow cannot exceed for given upstream conditions. The U.S. standard discloses equations that allow precise computation of the gas mass flow rate based on the nature of the gas, the throat diameter, the upstream pressure and temperature and certain thermodynamic characteristics of the flowing gas. In a critical flow nozzle, enough pressure is applied upstream from the nozzle throat relative to the downstream pressure to ensure that the gas flow velocity at the throat becomes critical, i.e., attains the speed of sound and cannot be increased further. The gas mass flow rate then becomes proportional to the density of the gas upstream of the nozzle, i.e., to the pressure upstream of the nozzle. The mass flow rate then can be precisely computed on the basis of the nature of the gas, the gas temperature and the pressure upstream of the nozzle.
The above mentioned U.S. standard describes use of critical flow nozzles to measure gas mass flow rates, and specifies or recommends all of the conditions for various uses of a critical flow nozzle, including the diameter of the upstream passage compared to the diameter of the nozzle, where the pressure connection should be located to read the upstream gas pressure, where the temperature probe should be positioned upstream of the nozzle, and various other parameters.
However, a problem of critical flow nozzles disclosed in the U.S. Standard is that for low gas flow rates, if the temperature of the incoming gas is substantially different than the ambient temperature, the difference can result in inaccurate measurement of the gas mass flow rate, because the temperature of the gas between the temperature probe and the critical flow nozzle is affected due to thermal conduction and radiation of the tube. Specifically, the U.S. standard teaches that the gas mass flow computations may be inaccurate if the temperature of the incoming gas is more than 5 degrees Centigrade different than the ambient temperature.
The U.S. standard recommends exact adherence to its published recommendations, and does not provide any suggestion of how more precise measurement of gas mass flow rates might be achieved.
Above mentioned U.S. Pat. No. 5,445,035 discloses a mass flow meter that includes a body having a cylindrical bore and an elongated cylindrical piston positioned in the bore concentrically with the body. An elongated annular fluid flow channel of uniform depth is bounded by a cylindrical surface of the piston and a surface of the bore, and the gas flows through the channel in the laminar flow regime. A first pressure measuring probe in fluid communication with an upstream equalization chamber measures fluid pressure in the upstream equalization chamber, and a second differential pressure transducer in fluid communication between the upstream equalization chamber and a downstream equalization chamber measures differential fluid pressure between the two equalization chambers. The difference between the pressures measured in the two equalization chambers represents the mass flow of the fluid through the channel. The simple, near-ideal geometric shapes of the bore, piston, and ferrules supporting the piston interact so as to permit simple, accurate mathematical modeling of corrections to account for changes in pressure, temperature, and thermal gradient. By design, the temperature of the gas in the flow path assumes the temperature of the body in which the bore is located so that the gas temperature can be determined by measuring the temperature of the body.
However, the mass flow meter described in U.S. Pat. No. 5,445,035 cannot provide accurate gas mass flow rate measurements at gas mass flow rates greater than approximately 30 standard liters per minute (slm). Even using the latest technology, the upper limit for gas flow rates that can be accurately measured using the laminar flow technology described in the ""035 patent is approximately 100 slm. This is because as flow rate increases, the velocity of the gas in the flow path increases, and the gas is not in the flow meter body for a sufficient amount of time for the gas to precisely assume the temperature of the body.
The setup and exploitation of critical flow nozzles to measure gas mass flows are precisely defined by the above U.S. standard. The objective of the setup recommendations in the above U.S. standard is to be able to predict the gas flow from the diameter of the throat of the nozzle by means of the recommended calculations. The calculations use tables of value of the critical flow function for various gases as a function of the pressure and temperature upstream from the nozzle and a calculation of the discharge coefficient as a function of the Reynolds number of the gas flow. The calculation of the discharge coefficient is applicable only for Reynolds numbers greater than 1*10. Those skilled in the art will understand that a lower limit Reynolds number prevents use of the recommended calculations for low gas flow rates.
The problem faced by the applicant was how to use the well-known principles described in the above prior art, especially the U.S. standard, to achieve more precise, and especially more repeatable, gas flow rate measurements in the range of flow from less than a standard liter per minute to 5000 standard liters per minute.
Thus, there remains an unmet need for an improved mass flow meter that is capable of making accurate gas flow measurements which are more precise and especially more repeatable than the prior art mass flow meters at any gas flow rate, especially at gas flow rates up to approximately 5000 standard liters per minute or more.
Accordingly, it is an object of the invention to provide a mass flow meter capable of providing accurate, precisely repeatable gas flow measurements over a very wide range.
It is another object of the invention to provide a mass flow meter capable of providing accurate, precisely repeatable gas flow measurements at flow rates up to approximately 5000 liters per minute.
It is another object of the invention to provide a single mass flow meter with a very wide useful range or xe2x80x9crangeabilityxe2x80x9d, i.e., 10:1.
It is another object of the invention to provide a mass flow meter capable of providing accurate, precisely repeatable gas flow measurements at high flow rates using a physical structure quite similar to that of the prior laminar flow mass flow meters described in U.S. Pat. No. 5,445,035.
It is another object of the invention to provide a mass flow meter capable of providing more accurate, more precisely repeatable gas flow measurements, even at low flow rates, than is possible using the mass flow meter of U.S. Pat. No. 5,445,035.
It is another object of the invention to provide a mass flow meter which avoids instability over time caused by contamination from the gas flow being measured.
It is another object of the invention to provide a mass flow meter which is not significantly affected by a difference between the temperature of gas entering the flow meter and the ambient temperature.
It is another object of the invention to provide a mass flow meter that is not significantly affected by the geometry of piping upstream of and connected to the fluid inlet of the flow meter.
It is another object of the invention to provide a mass flow meter that can be used with the mass flow terminal described in U.S. Pat. No. 5,445,035.
Briefly described, and in accordance with a xe2x80x9clow flowxe2x80x9d embodiment thereof, the invention provides an apparatus and method for measuring gas flow at relatively low gas flow rates by providing an elongated fluid flow channel extending through a high thermal mass body, a critical flow nozzle in a downstream portion of the channel, and a heat exchanger in the upstream portion of the channel in close thermal contact with the high thermal mass body. Gas is forced to flow into an upstream portion of the channel and through the heat exchanger at a sufficiently high pressure to ensure critical flow of the gas through the critical flow nozzle, wherein the heat exchanger brings a temperature of gas emerging from the heat exchanger to a value essentially equal to the temperature of the high thermal mass body. The pressure of the gas in the upstream portion of the channel is measured and is caused to sufficiently exceed the pressure of the gas downstream from the critical flow nozzle to ensure sonic flow of the gas through the critical flow nozzle. The temperature of a portion of the high thermal mass body adjacent to the upstream portion of the channel is measured. The mass flow rate of gas through the critical flow nozzle is computed from the upstream pressure, the temperature of the high thermal mass body, and a dimensional characteristic (i.e., throat diameter) of the critical flow nozzle. In this embodiment, the flow of the gas into the channel is in the range from 100 standard cubic centimeters per minute to approximately 100 standard liters per minute. A flow straightener is provided in the upstream portion of the channel integrally with the heat exchanger, wherein the flow straightener includes a plurality of holes radially oriented about the flow channel. A programmable read-only memory unit stores information for use in computing the mass flow rate for different gases through the critical flow nozzle. The mass flow rate through the critical flow nozzle is computed using values of discharge coefficients that have been experimentally determined for the same Reynolds number values for each type of gas. The diameter of a passage through the critical flow nozzle is in the range from approximately 0.2 millimeters to approximately 2 millimeters. The heat exchanger is located less than 5 times the diameter of the channel upstream of the nozzle from an upstream face of the critical flow nozzle. The amount of expansion of a diameter of the throat of the critical flow nozzle due to change in temperature of the critical flow nozzle is computed using a measured temperature of the high thermal mass body, and the computed amount of expansion due to the upstream pressure is used to correct the throat diameter deformation in computing the mass flow rate of the gas through the critical flow nozzle.
In a xe2x80x9chigh flowxe2x80x9d embodiment, an apparatus and method for measuring gas flow at relatively high flow rates includes providing an elongated fluid flow channel extending through a high thermal mass body, providing a critical flow nozzle in a downstream portion of the channel, and a heat exchanger in the upstream portion of the channel in close thermal contact with the high thermal mass body. Gas is forced to flow into an upstream portion of the channel and through the heat exchanger at a sufficiently high pressure to ensure critical flow of the gas through the critical flow nozzle. A heat exchanger is provided so as to bring the temperature of gas emerging from the heat exchanger very close to a temperature of the high thermal mass body. The pressure of the gas in the upstream portion of the channel is measured, and the pressure of the gas in the upstream portion of the channel is caused to sufficiently exceed a pressure of the gas downstream from the critical flow nozzle to ensure sonic flow of the gas through the critical flow nozzle. The temperature of gas emerging from the heat exchanger upstream from the critical flow nozzle is measured. The mass flow rate of gas through the critical flow nozzle is computed from the upstream pressure, the temperature of the gas, and a dimensional characteristic of the critical flow nozzle. In one described xe2x80x9chigh flowxe2x80x9d embodiment, the flow rate of the gas into the channel is in the range from 5 standard liters per minute to approximately 5000 standard liters per minute, although the various device dimensions and the range of flow rates of the gas into the channel for this embodiment of the invention are completely scalable. A flow straightener is provided integrally with the heat exchanger. A programmable read-only memory unit stores information for use in computing the mass flow rate through the critical flow nozzle using values of discharge coefficient that have been experimentally determined for each gas type. The passage through the critical flow nozzle is in the range from approximately 1 millimeter to approximately 10 or more millimeters. The heat exchanger is located less than 5 times the diameter of the channel upstream of the nozzle from an upstream face of the critical flow nozzle. An amount of expansion of a diameter of a throat of the critical flow nozzle due to a change in temperature of the critical flow nozzle is computed, and the computed amount of expansion due to the upstream pressure is used to correct the throat diameter deformation in computing a mass flow rate of gas through the critical flow nozzle.